How to Connect Batteries in Parallel

Rechargeable lithium batteries such as ours are widely used in various applications, from portable electronics to renewable energy systems. Connecting multiple lithium batteries in parallel can be a smart way to increase capacity and achieve longer-lasting power sources. However, doing this improperly can result in safety hazards and damage to the batteries. In this blog post, we'll guide you through the process of properly connecting lithium batteries in parallel while ensuring safety and efficiency.

Disclaimer: Always exercise caution when working with lithium batteries. Follow the manufacturer's guidelines and safety precautions, and consider seeking professional assistance if you are unsure about any step in the process.

Why Connect Batteries in Parallel?

Connecting multiple batteries in parallel is the easiest way to increase the capacity of your system without changing the voltage. The total capacity is simply the sum of all individual capacities. For example, connecting two of our 12-volt 100 amp-hour renewed batteries in parallel creates a system with 12-volt 200 amp-hour capacity (100 + 100 = 200).

Materials Needed:

Before you start, gather the following materials:

1. Lithium batteries with the same chemistry and voltage rating. Make sure the batteries are within 0.25 volts of one another to minimize the chances of sparking when connected! A large difference in voltage will create an unsafe condition where the battery with the higher state of charge (SOC) will try to charge those at lower SOC. Since lithium batteries have very low internal resistance, even a small difference in voltage can turn into high inrush current when connected, potentially damaging the batteries or causing a dangerous situation.
2. Personal protective equipment (PPE) such as gloves and safety goggles or a face shield.
3. Wiring and connectors (ensure they can handle the required current).
4. Insulating materials and heat shrink tubing to prevent short-circuit.

Step-by-Step Guide:

1. Safety First:

   Before you begin, make sure you are in a well-ventilated area and are wearing appropriate safety gear, including gloves and safety glasses. Keep the terminal protectors installed until you're ready to connect the batteries together.

2. Check Battery Compatibility:

   Ensure that the lithium batteries you intend to connect in parallel have the same voltage and SOC. Mixing batteries with different specifications can lead to imbalanced charging and discharging, which is unsafe. Batteries that are at different SOC should be charged or discharged to within 0.25 volts to prevent damage due to excessive current.

3. Connect the Batteries:

   Connect the batteries in parallel, which means that the positive terminals are connected to each other, and the negative terminals are connected to each other. You can use heavy-duty copper wire or bus bars to make these connections. Make sure to size your wires according to the maximum amperage that they will see. Here is a handy chart to select the correct size wire.

4. Use Insulation:

   To prevent accidental short circuits, insulate all exposed connections with insulating materials or heat shrink tubing. This will help protect your batteries and reduce the risk of electrical fires. Clamp the batteries using a battery hold-down if they are in a mobile application (RV, trailer, etc.).

5. Double-Check Connections:

   Before you power up your parallel-connected batteries, double-check all connections to ensure they are secure and correctly oriented. Loose connections can lead to inefficiencies and potentially dangerous situations.

6. Monitor and Test:

   Once your batteries are connected, monitor them closely, especially during the initial charging and discharging cycles. This will help you ensure that the batteries are working together harmoniously and that no issues arise.

Additional Resources:

For more detailed information, you can refer to the following resources:

1. Battery University - Parallel and Series Battery Configurations

   This resource provides an in-depth explanation of the advantages and disadvantages of connecting batteries in series and parallel.

2. DIY Lithium Battery Builder's Guide

   A community-driven guide on building lithium battery packs, including parallel connections.

3. How to Build a Lithium Battery

   This tutorial covers various aspects of building a lithium battery, including parallel connections.

Conclusion:

Properly connecting lithium batteries in parallel can be a beneficial way to increase capacity and enhance your power supply. However, safety should always be a top priority when working with lithium batteries. By following the steps outlined in this guide and the recommendations of your battery and BMS manufacturer, you can create a safe and efficient parallel battery configuration for your specific needs. Always monitor your batteries and stay informed about best practices to ensure the longevity and reliability of your power source.